7. Ulmus thomasii Sarg. (rock elm, cork elm)

Pl. 570 h–j; Map 2669

Plants trees to 30 m tall. Twigs tan to reddish brown, hairy when young, sometimes sparsely so, often becoming nearly glabrous with age, usually at least some older twigs developing irregular, corky wings anastomosing in several ranks. Winter buds 4–8 mm long, narrowly ovoid to narrowly ellipsoid, sharply pointed, brown to more commonly reddish brown, minutely pubescent with white to red hairs. Petioles 3–5(–7) mm long. Leaf blades 8.5–15.0 cm long, 4.5–9.5 cm wide, elliptic to more or less rhombic-elliptic, the base strongly asymmetric, short-tapered to a sharply pointed tip, the major marginal teeth 2–4 mm deep, sharp, all or most with 1–3 smaller secondary teeth, the upper surface smooth, the undersurface moderately hairy, more densely so along the main veins, sometimes also tufted in the vein axils, the secondary veins 15–29 on each side of the midvein, the lateral veins rarely forked toward their tips; juvenile leaves never lobed. Inflorescences short pendant racemes appearing before the leaves develop in the spring on second-year twigs. Flowers with the stalks 5–10 mm long, the calyces (5–)7- or 8-lobed to about the midpoint, the tube glabrous or sparsely pubescent with short straight hairs, the lobes broadly rounded, glabrous or with a few marginal hairs. Fruits 1.7–2.3 cm long, 1.1–1.3 cm wide, elliptic, tan, somewhat inflated, the body and wings evenly and finely hairy on the surfaces, the wing margins also densely hairy. 2n=28. April.

Uncommon in the northern and central portions of the state (Eastern North America, from southern Ontario and Minnesota south to Virginia and Arkansas). Bottomland forests, mesic upland forests, tops and ledges of bluffs, and rarely savannas.

Ulmus thomasii resembles U. americana in the form and serration of its leaves and in its long, narrowly pointed winter buds. When vegetative, the two species are easily confused. Ulmus thomasii is considered to have the best-quality wood of any of our elms, and it was extensively logged in the past. Likely it was never common in Missouri and many of the herbarium records documenting its distribution are historical.